ELECTROLYTIC CAPACITORS ตัวเก็บประจุชนิดอิเล็กทรอไลติก

Electrolytic capacitors are specified where high values of capacitance are required in the least amount of space (high volumetric efficiency). This property is called high CV ratio. They are formed by electrochemical processes in which oxide dielectrics are grown in and on porous aluminum and tantalum foil and pellets. The metal foils are acid etched to make them porous, increasing their effective exposed areas from 6 to 20 times. High CV ratios are made possible by the thin oxide layers formed on the plates of the capacitors. The pellets are also made so that they are porous or spongelike and have large exposed surfaces. However, electrolytic capacitors have higher leakage current than electrostatic capacitors because of the impurities embedded in the foil and the electrolyte. This current increases with temperature while voltage breakdown decreases with temperature. Electrolytic capacitors also have higher power factors than electrostatic capacitors, causing losses called equivalent series resistance (ESR).

MONOLITHIC MULTILAYER CERAMIC (MLC) CAPACITORS

A monolithic multilayer ceramic (MLC) capacitor, as shown in cutaway view Fig. 1-14, is a multilayer ceramic chip capacitor that offers high volumetric efficiency because a large capacitor area is compressed into a small block. Preformed metallized layers are stacked and fired to form MLCs in a wide range of sizes and values with different properties. Originally developed for hybrid circuits, MLCs are widely used in surface mounting because they can substitute for larger capacitors with comparable capacitance values. They offer low residual inductance values and low resistance, a wide range of capacitance values in a given size, and a wide selection of temperature coefficients. They also exhibit lower inductance and resistance values than tantalum capacitors with comparable ratings. MLCs are used for timing and frequency selection.



MLCs are made as sandwiches of “green” (unfired) barium-titanate ceramic strips 0.8 mils (20 μm) thick that have been imprinted with silver-palladium ink to form plates. Up to 40 layers of the soft doughlike strips are stacked, compressed, diced, and furnace fired to form the monolithic chips.

End terminals for solder bonding MLCs to a circuit board or attaching leads are made by plating successive layers of silver-palladium, nickel, and tin or lead-tin on the ends of the chips. The process used depends on whether the chip is to be leaded and coated with insulation or is to remain bare for bonding directly to a circuit board.

Bare MLCs are used on hybrid microcircuits and in surface-mount assembly. They will withstand the 232°C reflow-soldering temperatures and the 282°C wave-soldering temperatures. Bare MLC chip sizes are standardized. Examples include 0.08 × 0.05 in (2.0 × 1.3 mm), designated 0805; 0.125 × 0.063 in (3.2 × 1.6 mm), designated 1206; and 0.225 × 0.05 in (5.7 × 1.3 mm), designated 2225. Standard MLCs have capacitance values of 10 pF to 3.5 μF, capacitance tolerances of +- 1 to 20 percent, and maximum voltages of 50 V.

CERAMIC TUBULAR CAPACITORS

A ceramic tubular capacitor is a length of ceramic tube whose inner and outer surfaces are painted with silver ink to form its plates. They have replaced ceramic disk capacitors in surface-mounted circuits to save board space and permit automatic placement. They are protected with a coat of protective resin.

CERAMIC CAPACITORS เซรามิก คาปาซิเตอร์

Ceramic dielectric capacitors are classified by dielectric constant k, as Classes I, II, and III. Class I dielectrics exhibit low k values, but they have excellent temperature stability; Class II dielectrics have generally high k values and volumetric efficiency but lower temperature stability; and Class III dielectrics are prepared for the lower-cost disk and tube capacitors.



Class I dielectrics include negative positive zero (NPO) ceramics, which are designated COG and BY. These ceramics are made by combining magnesium titanate (with a positive coefficient) and calcium titanate (with a negative coefficient) to form a dielectric with excellent temperature stability. Their properties are essentially independent of frequency, and they have ultrastable temperature coefficients of 0 +- 30 ppm°C over the range of −55 to 125°C. These dielectrics show a flat response to both AC and DC voltage changes. Low-k multilayer ceramic capacitors (MLCs) are used in resonant circuits and filters.

Class II dielectrics are high-k ceramics called ferroelectrics made from barium titanate. The addition of barium stannate, barium zirconate, or magnesium titanate lowers the dielectric constant from values as high as 8000. These compounds stabilize the capacitor over a wider temperature range. Class II dielectrics include the general-purpose X7R (BX) and Z5U (BZ). X7R is stable but its capacitance can vary +-15 percent over the temperature range of −55 to 125°C. Its capacitance value decreases with DC voltage but increases with AC voltage. Z5U compositions exhibit maximum temperature-capacity changes of +22 and −56 percent over the range of 10 to 85°C.

Class III dielectrics, developed for ceramic-disk capacitors, give high volumetric efficiency but with the tradeoff of high leakage resistance and dissipation factor. Capacitors made with Class III dielectrics have low working voltages. Ceramic dielectric capacitors are constructed in three styles: (1) single-layer disk, (2) tubular, and (3) monolithic multilayer

MICA CAPACITORS ไมก้า คาปาซิเตอร์

A mica capacitor has dielectrics of thin rectangular sheets of mica, a natural mineral. Mica has a dielectric constant from 6 to 8. The electrodes are either thin sheets of metal foil interleaved between mica sheets, or thin films of silver that have been screened and fired on the mica. Silvered mica capacitors (ซิลเวอร์ไมก้าคาปาซิเตอร์ เหมาะที่จะใช้กับวงจรความถี่สูง) have greater mechanical stability and offer more uniform properties than foil and mica capacitors. Both are used primarily in RF applications. Mica capacitors perform satisfactorily over temperature ranges as wide as −55 to 150°C, and they have high insulation resistance. Their capacitance values range from about 1 pF to 0.1 μF. However, they have a low ratio of capacitance to volume or mass.

FILM DIELECTRICS

Polyester film (tradenamed Mylar) is the most popular general-purpose dielectric in filmtype capacitors. It permits smaller capacitors than comparably rated units made from other films, and these capacitors exhibit low leakage, moderate temperature coefficients over the −55 to 85°C range, and moderate dissipation factors. Capacitance tolerance is typically +-10 percent. The film-and-foil versions are widely used in consumer electronics products while the metallized units perform general blocking, coupling, decoupling, bypass, and filtering functions.



Polypropylene film provides capacitor characteristics that are superior to those of polyester. Polypropylene capacitors have both high- and low-frequency applications. The plastic has properties that are similar to those of polystyrene, but capacitors made from it have higher AC current ratings. Polypropylene capacitors can operate at 105°C, and their volumetric efficiency is better than those made of polyester. Foil and polypropylene capacitors are used in CRT deflection, pulse-forming, and RF circuits. The capacitance tolerance for polypropylene capacitors is +-5 percent, and their temperature coefficients are linear.

Polystyrene film has characteristics that are similar to those of polypropylene. Capacitors made from the film exhibit a low dissipation factor, small capacitance change with temperature, and very good stability. But they are larger than comparably rated polypropylene units. Used in timing, integrating, and tuning circuits, their maximum operating temperature is 85°C.

Polycarbonate film capacitors offer dissipation factors and capacitance stability which approaches those of polystyrene capacitors. They also offer high insulation resistance stability. Operating temperatures are −55 to 125°C with capacitance tolerances of +-5 percent. These capacitors are widely used in military applications.

PLASTIC-FILM CAPACITORS

A plastic-film capacitor, as shown in Fig. 1, is typically made by rolling a thin film of plastic dielectric with metal foil or a metallized dielectric film into a cylindrical form and attaching leads. The dielectrics include polyester, polypropylene, polystyrene, and polycarbonate. Film thickness can range from 0.06 mil (1.5 μm) to over 0.8 mil (20 μm). The most popular film capacitors have capacitance values of 0.001 to 10 μF, although values from 50 pF to 500 μF are available as standard products. Working voltages range from 50 to 1600 VDC, and capacitance tolerance is from +-1 to +-20 percent.



Fig. 1

In film-and-foil construction, tin or aluminum foil about 0.00025 in (0.00635 mm) thick is wound with the dielectric film, but in metallized-film construction, aluminum or zinc is vacuum deposited to thicknesses of 200 to 500 Å (20 to 50 nm) on the film. Film capacitors can also be made by cutting and stacking metallized foil with attached leads. A capacitor with metallized film is smaller and weighs less than a comparably rated film-and-foil unit. Moreover, metallized-film capacitors are self-healing; that is, if the capacitor dielectric is pierced by a transient overvoltage, the metal film around the hole will evaporate, effectively lining the hole with molten plastic dielectric. This prevents short-circuits between adjacent metal layers and preserves the capacitor.

After rolling or stacking is complete, the capacitor is dipped in or conformally coated with an insulating plastic jacket. Some units are also hermetically sealed in tubular or rectangular metal cases for added environmental protection. Both film-and-foil and metallizedfilm capacitors are available with axial or radial leads in a wide variety of case styles.

ELECTROSTATIC CAPACITORS

An electrostatic capacitor has a dielectric made from plastic film, mica, or glass, and its plates or electrodes are made from metal foil or metal deposited on the dielectric. Ceramic capacitors have plates formed from precious-metal inks that have been screened on the raw ceramic prior to furnace firing.

Capacitors,คาปาซิเตอร์, ตัวเก็บประจุ

A capacitor, as shown in Fig. 1, is an electronic component capable of storing electrical
energy. The simplest form of capacitor is two metal plates insulated from each other by some dielectric. Capacitors are the second most widely purchased passive components next to resistors. There are both fixed and variable capacitors for electronics, and their capacitance values vary from a few picofarads (pF) to thousands of microfarads (μF). The schematic symbol for a fixed capacitor is shown in Fig. 2 and that for a variable capacitor is shown in Fig. 3.




Fig. 1




Fig. 2


Fig. 3


Capacitors are classified as either electrostatic or electrolytic. Electrostatic capacitors have dielectrics that are either air or some solid insulating material such as plastic film, ceramic, glass, or mica. (Paper dielectric capacitors are no longer specified in electronics.)

Electrolytic capacitors are further classified as aluminum or tantalum because those metals form thin oxide film dielectrics by electrochemical processing. They can have wet-foil, wet-slug, or dry-slug anodes.

The capacitance value of fixed capacitors remains essentially unchanged except for small variations caused by temperature changes. By contrast, the capacitance value of variable capacitors can be set to any value within a preset range of values. Variable capacitors are usually used in RF circuits.

Variable Resistors ตัวต้านทานปรับค่าได้

POTENTIOMETERS

A potentiometer is a variable resistor whose resistance value can be changed by moving a sliding contact or wiper along its resistive element to pick off the desired value. A potentiometer has terminals at each end of its fixed resistive element, and the third terminal is connected to a moveable wiper. If the wiper is moved back to the beginning of the resistive element, the potentiometer’s resistance value is minimal, but if it is moved across the full length of the element, the value reaches its maximum. There are three different mechanisms for moving the wiper along the resistance element:



1. Sliding the wiper by finger pressure
2. Turning a leadscrew on the case to drive the wiper back and forth
3. Rotating a screw or knob attached to the wiper to sweep it around a curved element

Potentiometers for electronic circuits are classified as follows:

• Precision
• Panel or volume-control
• Trimmer

The common abbreviation for potentiometer is pot, so there is a precision pot and a panel or volume-control pot. However, a trimmer potentiometer is usually called a trimmer (to be distinguished from a trimmer capacitor). These variable resistors share the same schematic symbol and are made from many of the same kinds of materials.

Fixed Resistors ตัวต้านทานแบบค่าคงที่

A resistor is a circuit component that provides a fixed value of resistance in ohms to oppose the flow of electrical current. Resistors can limit the amount of current flowing in a circuit, provide a voltage drop in accordance with Ohm’s laws, or dissipate energy as heat.

Fixed resistors are discrete units typically made in cylindrical or planar form. The most common cylindrical style is the axial-leaded resistor, as shown in Fig. 1. The resistive element is wound or deposited on a cylindrical core, and a cap with a lead wire is positioned on each end. The resistive elements include resistive wire (wirewound), metal film, carbon film, cermet, and metal oxide. Resistor networks and chip resistors are examples of planar resistors. All fixed resistors are rated for a nominal resistance value in ohms over the range of fractions of an ohm to thousands of ohms (kilohms), or millions of ohms (megohms). Other electrical ratings include:



Figure 1

Some resistors also have additional ratings for electrical noise, parasitic inductance, and parasitic capacitance. Resistors exhibit unwanted parasitics of inductance and capacitance because of their construction. These effects must be considered by the designer when selecting resistors for unusual or specialized applications such as their use in instrumentation. A resistor’s ability to dissipate power is directly related to its size. With the exception of those specified for power supplies, most resistors for electronic circuits are rated under 5 W, usually less than 1 W. A 5-W cylindrical resistor is about 1 in (25.4 mm) long with a diameter of 1⁄4 in (6.4 mm). The 1⁄2-, 1⁄4-, and 1⁄8-W resistors are correspondingly smaller.

• Resistive tolerance as a percentage of nominal value in ohms
• Power dissipation in watts (W)
• Temperature coefficient (tempco) in parts per million per degree Celsius of temperature
• change (ppm/°C)
• Maximum working voltage in volts (V)

CARBON-COMPOSITION RESISTORS

A carbon-composition resistor, as shown in Fig. 2, is made by mixing powdered carbon with a phenolic binder to form a viscous bulk resistive material, which is placed in a mold with embedded lead ends and fired in a furnace. Because their resistive elements are a bulk material, they can both withstand wider temperature excursions and absorb higher electrical transients than either carbon- or metal-film resistors. These qualities are offset by their typically wider resistive tolerances of +-10 to 20 percent and tendency to absorb moisture in humid environments, causing their values to change. However, the benefits of carboncomposition resistors are less important in low-voltage transistorized circuits, so demand for them has declined. These resistors have ratings of 1 ohm to 100 megohms, but values in the 10- to 100-ohm range were most popular. Power ratings are 1⁄8 to 2 W.



Figure 2

CARBON-FILM RESISTORS

A carbon-film resistor, as shown in Fig.3, is made by screening carbon-based resistive ink on long ceramic rods or mandrels and then firing them in a furnace. The rod is then sliced to form individual resistors. After leaded end caps are attached, the resistance values are set precisely in a laser trimming machine that trims away excess resistive film under closed-loop control. The trimmed resistors are then coated with an insulating plastic jacket. Resistive tolerances of carbon-film resistors are typically +-10 percent. Standard resistors have power ratings of 1⁄2, 1⁄4, and 1⁄8 W.



Figure 3

WIREWOUND RESISTORS

A wirewound resistor, as shown in Fig.4, is made by winding fine resistive wire on a plastic or ceramic mandrel. The most commonly used resistance wire is nickel-chromium (nichrome). The axial leads and end caps are attached to the ends of the wire winding and welded to complete the electrical circuit. There are both general-purpose and power wirewound resistors. General-purpose units have resistive values of 10 ohms to 1 megohm, resistance tolerances of +-2 percent, and temperature coefficients of +-100 ppm/°C. Power
units rated for more than 5 W have tolerances that can exceed +-10 percent.



Figure 4

Wirewound resistors are generally limited to low-frequency applications because each is a solenoid that exhibits inductive reactance in an AC circuit, which adds to its DC resistive value. The inductive reactance can be reduced or eliminated at low or medium frequencies by bifilar winding. This is done by folding the entire length of resistive wire back on itself, hairpin fashion, before winding it on the mandrel. As a result, opposing inductive fields cancel each other, lowering or eliminating inductive reactance.

Wirewound resistors are made with both axial and radial leads. Epoxy or silicone insulation is applied to some low-power wirewound resistors, but high-power units are encased in ceramic or placed in heat-dissipating aluminum cases. This reduces the danger of the hot resistor igniting nearby flammable materials or burning fingertips if accidentally touched.



Figure 5

METAL-FILM RESISTORS

A metal-film resistor, as shown in Fig. 5, is made by the same general method as a carbon-film resistor. A thin metal film is sputtered or vacuum deposited on an alumina (aluminum-oxide) mandrel in a vacuum chamber, or a thick metal film is applied in air. Tin oxide or nickel-chromium are widely used thin films, and a thick film made from powdered precious metal and glass (frit) in a volatile binder is a common cermet resistive ink. These resistors are laser trimmed to precise values under closed-loop control after firing. Metal-film resistors are offered in two grades: (1) those with resistive tolerances of +-1 percent and temperature coefficients of 25 to 100 ppm/°C, and (2) those with resistive tolerances of +-5 percent and temperature coefficients of 200 ppm/°C. Demand is highest for 1⁄4- and 1⁄8-W units, but 1⁄20-W units are available. Resistive values up to 100 megohms are available as catalog items, but they are generally rated for less than 10 kilohms.


Figure 6

RESISTOR NETWORKS

A resistor network, as shown in Fig. 6, consists of two or more resistive elements on the same insulating substrate. These networks are specified where 6 to 15 low-value resistors are required in a restricted space. Most commercial networks contain thick-film resistors, and they are packaged in dual-in-line packages (DIPs) or single-in-line packages (SIPs). Standard DIPs have 14 or 16 pins, and standard SIPs have 6, 8, or 10 pins. Resistor networks are used for “pull-up” and “pull-down” transitions between logic circuits operating at different voltage values, for sense amplifier termination, and for light-emitting diode (LED) display current limiting.

Alumina ceramic is the most widely used network substrate. Conductive traces are formed by screening an ink made from a powdered silver-palladium mix in a volatile binder on the bare ceramic substrate. After firing, the ink bonds with the ceramic to form hard, lowresistance paths. Resistive inks made from a powdered ruthenium-cermet mix with a powdered glass frit and a volatile binder are then screened over the ends of the conductors to form the resistive elements. This ink is also fired, and when it bonds with the ceramic it forms a hard, resistive element. Network resistors are laser trimmed under closed-loop control to precise resistance values. Standard network resistance values are from 10 ohms to 10 megohms with tolerances of +-2 percent. Most networks can safely dissipate less than 1⁄2 W.

Where more precise resistance values are required, thin-film networks are specified. They are made formed from compositions that include nickel-chromium, chrome-cobalt, and tantalum nitride, deposited or sputtered on alumina ceramic substrates. Unpackaged thin-film resistor networks are also sold as hybrid-circuit substrates. Thin-film resistivecapacitive (RC) networks are also packaged in metal and ceramic flatpacks.



Figure 7

CERAMIC-CHIP RESISTORS

A ceramic-chip resistor, as shown in Fig. 7, is made by screening and firing cermet resistive inks or sputtering tantalum nitride or nickel-chromium on an alumina substrate. The deposited resistive surface is then coated with glass for protection. The substrate is then diced into individual chips, and a silver-based ink is applied to the end surfaces and fired as the first step in forming leadless terminals. A barrier layer of nickel plating is then applied to prevent the migration of silver from the inner electrode. Finally, the terminations are coated with lead-tin solder for improved adhesion during reflow soldering.

Chip resistors were originally made for hybrid circuits, but surface-mount technology (SMT) has increased demand for them. Surface-mount chip resistor dimensions have been standardized to 1.6 × 3.2 mm for handling by automatic pick-and-place machines. (This is the same size as the 1206 chip capacitor that measures 0.063 × 0.125 in.) Chip resistors are typically rated for 1⁄8 W or less. An alternative form of SMT resistor is the leadless cylinder with solder-coated bands around each end for reflow solder bonding.

Magnetism

Electric currents and magnetic fields are closely related. Whenever an electric current flows—that is, when charge carriers move—a magnetic field accompanies the current. In a straight wire that carries electrical current, magnetic lines of flux surround the wire in circles, with the wire at the center, as shown in Fig.1. (The lines of flux aren’t physical objects; this is just a convenient way to represent the magnetic field.) You’ll sometimes hear or read about a certain number of flux lines per unit cross-sectional area, such as 100 lines per square centimeter. This is a relative way of talking about the intensity of the magnetic field.


Fig. 1. Magnetic flux lines around a straight, current-carrying wire. The arrows indicate current flow.

Magnetic fields are produced when the atoms of certain materials align themselves. Iron is the
most common metal that has this property. The atoms of iron in the core of the earth have become aligned to some extent; this is a complex interaction caused by the rotation of our planet and its motion with respect to the magnetic field of the sun. The magnetic field surrounding the earth is responsible for various effects, such as the concentration of charged particles that you see as the aurora borealis just after a solar eruption.

When a wire is coiled up, the resulting magnetic flux takes a shape similar to the flux field surrounding the earth, or the flux field around a bar magnet. Two well-defined magnetic poles develop, as shown in Fig. 2.



Fig. 2. Magnetic flux lines around a current-carrying coil of wire. The flux lines converge at the magnetic poles.

Power and the Watt กำลังงานและวัตต์

Whenever current flows through a resistance, heat results. The heat can be measured in watts (symbolized W) and represents electrical power. (As a variable quantity in equations, power is denoted by the uppercase italic letter P.) Power can be manifested in many forms, such as mechanical motion, radio waves, visible light, or noise. But heat is always present, in addition to any other form of power, in an electrical or electronic device. This is because no equipment is 100 percent efficient. Some power always goes to waste, and this waste is almost all in the form of heat.



Fig. 1. Whenever current passes through a component having resistance, a voltage exists across that component.

Look again at Fig. 1. There is a certain voltage across the resistor, not specifically indicated.
There’s also a current flowing through the resistance, and it is not quantified in the diagram, either. Suppose we call the voltage E and the current I, in volts (V) and amperes (A), respectively. Then the power in watts dissipated by the resistance, call it P, is the product of the voltage in volts and the current in amperes:

P = EI

If the voltage E across the resistance is caused by two flashlight cells in series, giving 3 V, and if the current I through the resistance (a light bulb, perhaps) is 0.1 A, then E = 3 V and I = 0.1 A, and we can calculate the power P in watts as follows:


P = EI = 3 × 0.1 = 0.3 W

Suppose the voltage is 117 V, and the current is 855 mA. To calculate the power, we must convert the current into amperes: 855 mA = 855/1000 A = 0.855 A. Then:

P = EI = 117 × 0.855 = 100 W

Conductance and the Siemens

Electricians and electrical engineers sometimes talk about the conductance of a material, rather than about its resistance. The standard unit of conductance is the siemens, abbreviated S. When a component has a conductance of 1 S, its resistance is 1 Ω. If the resistance is doubled, the conductance is cut in half, and vice versa. Therefore, conductance is the reciprocal of resistance.

If you know the resistance of a component or circuit in ohms, you can get the conductance in
siemens: divide 1 by the resistance. If you know the conductance in siemens, you can get the resistance: divide 1 by the conductance. Resistance, as a variable quantity, is denoted by an italicized, uppercase letter R. Conductance, as a variable quantity, is denoted as an italicized, uppercase letter G. If we express R in ohms and G in siemens, then the following two equations describe their relationship:

G = 1/R
R = 1/G

Units of conductance much smaller than the siemens are often used. A resistance of 1 kΩ is
equal to 1 millisiemens (1 mS). If the resistance is 1 MΩ, the conductance is one microsiemens (1 μS). You’ll sometimes hear about kilosiemens (kS) or megasiemens (MS), representing resistances of 0.001 Ω and 0.000001 Ω (a thousandth of an ohm and a millionth of an ohm, respectively). Short lengths of heavy wire have conductance values in the range of kilosiemens. Heavy metal rods can have conductances in the megasiemens range.

Suppose a component has a resistance of 50 Ω. Then its conductance, in siemens, is 1/50 S,
which is equal to 0.02 S. We can call this 20 mS. Or imagine a piece of wire with a conductance
of 20 S. Its resistance is 1/20 Ω, which is equal to 0.05 Ω. You will not often hear the term milliohm. But you could say that this wire has a resistance of 50 mΩ, and you would be technically
right.

Determining conductivity is tricky. If wire has a resistivity of 10 Ω/km, you can’t say that it has a conductivity of 1/10, or 0.1, S/km. It is true that a kilometer of such wire has a conductance of 0.1 S, but 2 km of the wire has a resistance of 20 Ω (because there is twice as much wire). That is not twice the conductance, but half. If you say that the conductivity of the wire is 0.1 S/km, then you might be tempted to say that 2 km of the wire has 0.2 S of conductance. That would be a mistake! Conductance decreases with increasing wire length.

Figure 1 illustrates the resistance and conductance values for various lengths of wire having a
resistivity of 10 Ω/km.



Figure 1 Resistance and conductance for various lengths of wire having a resistivity of 10 ohms per kilometer.

Resistance and the Ohm

Resistance is a measure of the opposition that a circuit offers to the flow of electric current. You can compare it to the diameter of a hose. In fact, for metal wire, this is an excellent analogy: smalldiameter wire has high resistance (a lot of opposition to current), and large-diameter wire has low resistance (not much opposition to current). The type of metal makes a difference too. For example, steel wire has higher resistance for a given diameter than copper wire.

The standard unit of resistance is the ohm. This is sometimes symbolized by the uppercase Greek letter omega (Ω). You’ll sometimes hear about kilohms (symbolized k or kΩ), where 1 kΩ = 1000 Ω, or about megohms (symbolized M or MΩ), where 1 MΩ = 1000 kΩ = 1,000,000 Ω.

Electric wire is sometimes rated for resistivity. The standard unit for this purpose is the ohm per foot (ohm/ft or Ω/ft) or the ohm per meter (ohm/m or Ω/m). You might also come across the unit ohm per kilometer (ohm/km or Ω/km). Table 1 shows the resistivity for various common sizes of solid copper wire at room temperature, as a function of the wire size as defined by a scheme known as the American Wire Gauge (AWG).



Table 1. Approximate resistivity at room temperature for solid copper wire as a function of the wire size in American Wire Gauge (AWG).

When 1 V is placed across 1 Ω of resistance, assuming that the power supply can deliver an unlimited number of charge carriers, there is a current of 1 A. If the resistance is doubled to 2 Ω, the current decreases to 0.5 A. If the resistance is cut by a factor of 5 to 0.2 Ω, the current increases by the same factor, to 5 A. The current flow, for a constant voltage, is said to be inversely proportional to the resistance. Figure 1 is a graph that shows various currents, through various resistances, given a constant voltage of 1 V across the whole resistance.


Fig. 1 Current as a function of resistance through an electric device for a constant voltage of 1 V.

Resistance has another property. If there is a current flowing through a resistive material, there
is always a potential difference across the resistive component (called a resistor). This is shown in Fig. 2. In general, this voltage is directly proportional to the current through the resistor. This behavior of resistors is useful in the design of electronic circuits, as you will learn later.

Electrical circuits always have some resistance. There is no such thing as a perfect conductor.
When some metals are chilled to temperatures near absolute zero, they lose practically all of their resistance,but they never become absolutely perfect, resistance-free conductors. This phenomenon, about which you might have heard, is called superconductivity.



Fig. 2 Whenever current passes through a component having resistance, a voltage exists across that component.

Just as there is no such thing as a perfectly resistance-free substance, there isn’t a truly infinite resistance, either. Even air conducts to some extent, although the effect is usually so small that it can be ignored. In some electronic applications, materials are selected on the basis of how “nearly infinite” their resistance is.

In electronics, the resistance of a component often varies, depending on the conditions under which it is operated. A transistor, for example, might have high resistance some of the time, and low resistance at other times. High/low resistance variations can be made to take place thousands, millions, or billions of times each second. In this way, oscillators, amplifiers, and digital devices function in radio receivers and transmitters, telephone networks, digital computers, and satellite links (to name just a few applications).

The Ampere

Current is a measure of the rate at which charge carriers flow. The standard unit is the ampere. This represents one coulomb (6,240,000,000,000,000,000) of charge carriers flowing every second past a given point.

An ampere is a comparatively large amount of current. The abbreviation is A. Often, current is specified in terms of milliamperes, abbreviated mA, where 1 mA = 0.001 A, or a thousandth of an ampere. You will also sometimes hear of microamperes (μA), where 1 μA = 0.000001 A or 0.001 mA, which is a millionth of an ampere. It is increasingly common to hear about nanoamperes (nA), where 1 nA = 0.001 μA = 0.000000001 A, which is a thousandth of a millionth of an ampere.

A current of a few milliamperes will give you a startling shock. About 50 mA will jolt you severely, and 100 mA can cause death if it flows through your chest cavity. An ordinary 100-watt light bulb draws about 1 A of current in a household utility circuit. An electric iron draws approximately 10 A; an entire household normally uses between 10 and 50 A, depending on the size of the house and the kinds of appliances it has, and also on the time of day, week, or year.

The amount of current that flows in an electrical circuit depends on the voltage, and also on the
resistance. There are some circuits in which extremely large currents, say 1000 A, can flow. This will happen through a metal bar placed directly at the output of a massive electric generator. The resistance is extremely low in this case, and the generator is capable of driving huge numbers of charge carriers through the bar every second. In some semiconductor electronic devices, such as microcomputers, a few nanoamperes will suffice for many complicated processes. Some electronic clocks draw so little current that their batteries last as long as they would if left on the shelf without being put to any use.

Current Flow

If a conducting or semiconducting path is provided between two poles having a potential difference, charge carriers flow in an attempt to equalize the charge between the poles. This flow of current continues as long as the path is provided, and as long as there is a charge difference between the poles.

Sometimes the charge difference is equalized after a short while. This is the case, for example, when you touch a radiator after shuffling around on the carpet while wearing hard-soled shoes. It is also true in a lightning stroke. In these instances, the charge is equalized in a fraction of a second. In other cases, the charge takes longer to be used up. This happens if you short-circuit a dry cell. Within a few minutes, the cell “runs out of juice” if you put a wire between the positive and negative terminals. If you put a bulb across the cell, say with a flashlight, it takes an hour or two for the charge difference to drop to zero.

In household electric circuits, the charge difference is never equalized, unless there’s a power
failure. Of course, if you short-circuit an outlet (don’t!), the fuse or breaker will blow or trip, and the charge difference will immediately drop to zero. But if you put a 100-watt bulb at the outlet, the charge difference will be maintained as the current flows. The power plant can keep a potential difference across a lot of light bulbs indefinitely.



Figure 2 Relative current as a function of relative voltage for low, medium, and high resistances.

Have you heard that it is current, not voltage, that kills? This is a literal truth, but it plays on semantics. It’s like saying “It’s the heat, not the fire, that burns you.” Naturally! But there can only be a deadly current if there is enough voltage to drive it through your body. You don’t have to worry when handling flashlight cells, but you’d better be extremely careful around household utility circuits. A voltage of 1.2 to 1.7 V can’t normally pump a dangerous current through you, but a voltage of 117 V almost always can.

In an electric circuit that always conducts equally well, the current is directly proportional to
the applied voltage. If you double the voltage, you double the current. If the voltage is cut in half,
the current is cut in half too. Figure 2 shows this relationship as a graph in general terms. It assumes that the power supply can provide the necessary number of charge carriers.

Electrical Units

The Volt
An accumulation of electrostatic charge, such as an excess or shortage of electrons, is always associated with a voltage. There are other situations in which voltages exist. Voltage can be generated at a power plant, produced in an electrochemical reaction, or caused by light rays striking a semiconductor chip. It can be produced when an object is moved in a magnetic field, or is placed in a fluctuating magnetic field.

A potential difference between two points produces an electric field, represented by electric lines of flux (Fig.1). There is a pole that is relatively positive, with fewer electrons, and one that is relatively negative, with more electrons. The positive pole does not necessarily have a deficiency of electrons compared with neutral objects, and the negative pole does not always have a surplus of electrons relative to neutral objects. But the negative pole always has more electrons than the positive pole.



Electric lines of flux always exist near poles of electric charge.

The abbreviation for volt (or volts) is V. Sometimes, smaller units are used. The millivolt (mV) is equal to a thousandth (0.001) of a volt. The microvolt (μV) is equal to a millionth (0.000001) of a volt. It is sometimes necessary to use units larger than the volt. One kilovolt (kV) is one thousand volts (1000 V). One megavolt (MV) is 1 million volts (1,000,000 V) or one thousand kilovolts (1000 kV).

In a dry cell, the voltage is usually between 1.2 and 1.7 V; in a car battery, it is 12 to 14 V. In household utility wiring, it is a low-frequency alternating current of about 117 V for electric lights and most appliances, and 234 V for a washing machine, dryer, oven, or stove. In television sets, transformers convert 117 V to around 450 V for the operation of the picture tube. In some broadcast transmitters, the voltage can be several kilovolts.

The largest voltages on our planet occur between clouds, or between clouds and the ground, in thundershowers. This potential difference can build up to several tens of megavolts. The existence of a voltage always means that charge carriers, which are electrons in a conventional circuit, flow between two points if a conductive path is provided. Voltage represents the driving force that impels charge carriers to move. If all other factors are held constant, high voltages produce a faster flow of charge carriers, and therefore larger currents, than low voltages. But that’s an oversimplification in most real-life scenarios, where other factors are hardly ever constant!